1. Field of the Invention
The present invention relates to lithographic printing plates, presensitized photosensitive sheets capable of providing the lithographic printing plates and the process for preparing the lithographic printing plates from the presensitized photosensitive sheet material. More particularly, the present invention relates to lithographic printing plates in which the polymerized oleophilic polymer printing portions are more firmly adhered to the lithographic plate support by an anchoring layer formed from an hydrophobic oleophilic ester colloid which is capable of being rendered hydrophilic by an aqueous alkali solution. The present invention also relates to a process for preparing a lithographic printing plate from a presensitized photosensitive sheet in which a photosensitive layer of a photo-polymerizable composition overlying the anchoring layer is imagewise exposed to actinic radiation.
2. Discussion of the Prior Art
There have recently been introduced many photo-polymerizable compositions having improved mechanical characteristics and which can easily be applied as layers to various support materials. These photosensitive polymers which are oleophilic (ink receptive) have been used as the printing layers of lithographic printing plates. However, the use of these photo-polymerizable compositions has met with various drawbacks, particularly the difficulty of firmly adhering the printing layer, both before and after polymerization, to the support. Accordingly, there have been many proposals to improve the adherence of the photosensitive layer, including the polymerized and hardened photosensitive layer after image-wise exposure, to the support for the lithographic plate. These approaches generally take one or more of the following approaches:
modifying the support layer itself;
providing an anchoring layer between the support and the photosensitive layer;
incorporating adhesion promoting additives to the photosensitive layer, etc.
An example of the first mentioned technique is described in French Pat. No. 1,591,933. According to this patent a support of anodized aluminum is treated with an aqueous phosphoric acid to provide the anodized aluminum film with better porosity to thereby permit a better fixing of the photosensitive resin on the support. The treated support also has better water receptivity, which is a desirable objective for this type of plate. However, the process of the French Patent has the drawbacks that only a very special support, anodized aluminum, can be used and that the chemical treatment to which this support is subjected is rather long.
An example of the second category is described in U.S. Pat. No. 2,791,504 to L. Plambeck, Jr. The patentee discloses the use of a separate anchor layer formed from a compatible resin or film-forming polymer which is strongly adherent to both the support and the photopolymerized layer. According to this reference, the resins or polymers which have been used as carriers in the manufacture of paints, varnishes and lacquers are satisfactory. Resins or polymers which can be insolubilized or set up by controlled cross-linking with the aid of heat alone or in the presence of a catalyst or by a catalyst alone can also be used. A further class of resins or polymers are formed from the same monomers used in the photopolymerizable layers. Hydrophobic polyvinyl acetals are also described as suitable polymers. In Example I of this patent, a coating composition containing polyvinyl n-butyral is used as the anchor layer.
The third technique, namely incorporation of adhesion promoters to the photosensitive composition itself, is described in U.S. Pat. No. 3,820,993 to J. M. Lewis, et al. Specific adhesion promoters are listed in Table 7 in column 11, of the patent, all of the materials listed are organic sulfur compounds. This patent also lists various techniques for treating the support directly to improve adhesion.
While these different approaches provide differing degrees of adherence between the photopolymerizable layer and the support, still further improvements are desired. Furthermore, none of these prior art techniques consider modifying the bared regions of the anchoring layer to render these regions hydrophilic while the regions of the anchoring layer under the printing zones, i.e. hardened oleophilic photopolymers, resin hydrophobic and oleophilic.